CROSS Safety Report
Wind problem in city centre
This report is over 2 years old
A reporter shares how they have experienced a spate of wind related incidents where structures which have been designed in accordance with normal practice using current codes have suffered damage.
They feel that this is an issue which needs to be investigated and discussed so that engineers dealing with structures in the vicinity of tall buildings and groups of tall buildings are fully informed.
Key Learning Outcomes
For civil and structural design engineers:
Be aware of local wind effects and how suction pressures can be enhanced at corners, in gaps and on leeward faces which can affect finishes, glazing and cladding
The Standing Committee on Structural Safety (SCOSS) recently published the Alert on Wind adjacent to tall buildings which includes references for further information
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A reporter is dealing with temporary works for sub-contractors working in and around the centre of a major city. They have experienced a spate of wind related incidents where structures which have been designed in accordance with normal practice using current codes have suffered damage. It is difficult to ascertain the real cause of the problems, but they have experienced local winds in the same area which have been strong enough to actually lift people off their feet.
Design wind speeds exceeded
They feel that there is an issue which needs to be investigated and discussed so that engineers dealing with structures in the vicinity of tall buildings and groups of tall buildings are fully informed. From their observations it appears that winds at low level can exceed the design wind speeds by around 100-120%. This seems to be the case when downdraughts and vortices combine with ambient winds to produce loadings from both funnelled winds and opposing winds acting on the same element of a minor structure.
In a simple case, an arched tunnel seems to have experienced internal wind loads which can only have come about from winds acting on opposite ends of the tunnel which are at ninety degrees to each other, producing maximum inward pressure at one end and maximum suction at the other. In this case, the apparent increase in wind speed has been significant, resulting in suction loads on the tunnel walls with the capacity to pull out doors and windows.
Are local wind effects on surrounding buildings being assessed?
Clearly, tall buildings are designed using wind tunnels to ensure their stability and, the reporter assumes, to a lesser degree, the comfort of the public local to them. They doubt that the very local effects on surrounding buildings and structures are investigated in great depth but there must be some documentation that can be made available. EN1991-1-4 focuses on the primary structure but gives little guidance on how to disseminate information on the secondary effects for other structures in the vicinity.
Vortex shedding is covered but once the vortices are free of the structure, they become of no real interest to the building designer. As they can exist several hundreds of metres downwind and will tend to drop gradually to the ground, they can be of enormous interest to designers of buildings and elements of buildings over a large area. Unfortunately, the reporter thinks that these designers may not be aware of them until their structures fail. This, he says, needs addressing.
As they can exist several hundreds of metres downwind and will tend to drop gradually to the ground, they can be of enormous interest to designers of buildings and elements of buildings over a large area.
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Expert Panel Comments
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Wind around buildings is a complex issue and is affected not only by existing structures but by future development. Wind tunnel tests on new buildings in city centres are common but cannot predict everything accurately due to scale effects and the density of receptors on the models. Where there are clusters of very tall buildings there is a need for city wide modelling which can account for proposed development and effects some way downwind.
Who should take responsibility for this?
Government cuts have hit local authority budgets and there is no straightforward way to identify wind issues in relation to an individual building. Designers can only assess what is reasonably practicable and this is a difficult area but one that needs more attention perhaps than has been shown in the past. There is also the interface with planners to be considered.
If an event is reasonably foreseeable then designers would generally be expected to take it into account in their design. It is part of the designer’s implied professional duty to exercise reasonable skill and care. Where the effect has potential health and safety implications it is a statutory duty. The difficulty is that the designer will not know what developments may be proposed in the future and can only make a reasonable judgement making sure this is recorded and relayed to the client.
If an event is reasonably foreseeable then designers would generally be expected to take it into account in their design. It is part of the designer’s implied professional duty to exercise reasonable skill and care.
Climate change and wind patterns
Furthermore, climate change may result not only in different overall wind patterns but in extreme local effects such as mini tornados. As the reporter says more guidance is needed.
This was one of the reports which resulted in the recent Standing Committee on Structural Safety (SCOSS) publication on Wind adjacent to tall buildings which includes references for further information. An interesting article on the subject has been published by the BBC: The problem with the skyscraper wind effect